1. Field of the Invention
The present invention relates to poly(imide-benzoxazole) (PIBO) copolymers, to precursors of PIBO, and to films, coatings, fibers, adhesives and engineering plastics of PIBO.
2. Description of the Related Arts
Semiconductors are making rapid progress in capacity, density, integration and surface mounting. Along with this progress, a number of problems are arising in production of semiconductor devices. One of the problems is that the heat or thermal stress applied to a semiconductor chip at the time of surface mounting becomes more severe as the sealing material becomes thinner. Hence, it is necessary to protect the fine semiconductor circuits from heat or thermal stress. Meanwhile, in order to form circuits of high density and integration on a semiconductor chip, the technique of multi-layer wiring is indispensable. To achieve it, there is required an interlayer dielectric having high heat resistance, high adhesion and low dielectric constant.
It is known that the above problems can be solved by using aromatic polyimide (PI) or polybenzoxazole (PBO) as a passivation film, a buffer coat film, or an interlayer heterocyclic polymers with excellent thermal stability, high mechanical properties, good electrical properties, and superior chemical resistance. Films of polybenzoxazole here a lower moisture uptake than that of polyimide, but poor adhesion to silicon wafers, thus causing peeling problems. To improve the adhesion, polybenzoxazole can be modified by the incorporation of polyimide to form a polybenzoxazole-polyimidie copolymer.
The synthesis of the poly(imide-benzoxazole) (PIBO) copolymers has been disclosed in U.S. Pat. No. 5,071,948 to Khanna, and U.S. Pat. No. 5,985,969 to Harris et al. In these patents, PIBO copolymers are prepared through the condensation reaction of a pre-synthesized diamine monomer containing benzoxazole moiety with a dianhydride monomer or a pre-synthesized diacid chloride monomer containing imide moiety with a bis(o-aminophenol) monomer. These synthetic methods involve complex monomer synthesis and purification procedures, which are tedious and expensive. It also should be noted that none of these patents disclose a PIBO copolymer that contains hydroxyl (xe2x80x94OH) groups to improve adhesion to semiconductor substrates.
In view of the above disadvantages, an object of the invention is to provide a poly(imide-benzoxazole) (PIBO) copolymer which can be prepared directly from commercially available monomers.
Another object of the invention is to provide a PIBO copolymer that contains hydroxyl groups and a PIBO film produced therefrom that exhibits improved adhesion to semiconductor substrates.
A further object of the invention is to provide a precursor of the above PIBO copolymer and a precursor solution containing such precursor.
To achieve the above objects, there is provided a precursor of poly(imide-benzoxazole) prepared by polycondensation of a trimellitic anhydride halide monomer and a bis(o-diaminophenol) monomer, which comprises the following recurring unit: 
wherein
X is xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94C(CF3)2xe2x80x94, xe2x80x94C(CH3)2xe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94CH2xe2x80x94, xe2x80x94SO2xe2x80x94, xe2x80x94SOxe2x80x94, or deleted; and
n is an integer, generally from about 10 to about 1000.
By heating the above precursor at temperatures sufficient to effect cyclodehydration, a PIBO copolymer is afforded with the following recurring unit: 
wherein X and n are as set forth above.
In this invention, PIBO copolymers can be prepared directly from commercially available monomers in a one-pot reaction.
In the present invention, poly(imide-benzoxazole) copolymers are prepared directly from a trimellitic anhydride halide monomer and a bis(o-diaminophenol) monomer. A representative synthetic scheme is illustrated below: 
wherein X is xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94C(CF3)2xe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94CH2xe2x80x94, xe2x80x94SO2xe2x80x94, xe2x80x94SOxe2x80x94, or deleted; and n is an integer generally from about 10 to about 1000, preferably from 20 to about 200.
A poly(hydroxyamide-amic acid) precursor is first synthesized by low-temperatures solution polymerization in an organic solvent. Preferably, the bis(o-diaminophenol) monomer is dissolved in an organic solvent first and the trimellitic anhydride halide monomer is added thereto to react with the bis(o-diaminophenol) monomer. It is found that a higher inherent viscosity can be obtained when the trimellitic anhydride halide is added in a solid state than in a solution state. This may be due to the trimellitic anhydride halide""s moisture sensitivity reacting with moisture in the solvent and in the atmosphere. Organic solvents useful in this reaction include N-methylpyrrolidinone (NMP), pyridine, acetone, tetrahydrofuran (THF), dimethyl formamide (DMF), and mixtures thereof. The reaction is preferably conducted under substantially anhydrous conditions and at a temperature from about 15xc2x0 C. to xe2x88x9215xc2x0 C., preferably from about 0xc2x0 C. to xe2x88x925xc2x0 C. The most preferred trimellitic anhydride halide monomer is trimellitic anhydride chloride, and the most preferred bis(o-diaminophenol) monomer is 2,2-bis (3-amino-4-hydroxyphenol) hexafluoropropane (i.e., X=xe2x80x94C(CF3)2xe2x80x94), which are both commercially available.
In addition to the trimellitic anhydride halide and bis(o-diaminophenol), the polymer precursor may optionally be prepared using one or more other monomers, for example, a diacid dichloride such as terephthaloyl chloride or M-phthaloyl chloride. The optionally employed monomers may be added to the reaction mixture prior to or during the reaction of the trimellitic anhydride halide and bis(o-diaminophenol). A representative synthetic scheme including coplymerization of a diacid dichloride is as follows: 
wherein
X and n are as set forth above,
Ar is 
where Y is xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94C(CF3)2xe2x80x94, xe2x80x94C(CH3)2xe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94CH2xe2x80x94, xe2x80x94SO2xe2x80x94, or xe2x80x94SOxe2x80x94; and
m is an integer, and m+n is generally from about 10 to about 1000, preferably from about 20 to about 200.
Following preparation of the poly(hydroxyamide-amic acid) precursor, the precursor, generally without subsequent isolation, is exposed to conditions to afford the corresponding poly(imide-benzoxazole). In general, the polyimide precursor is simply heated to a temperature sufficient to cause dehydrative cyclization to form the benzoxazole ring and the imide ring. The temperature is preferably in a range from about 100xc2x0 C. to 450xc2x0 C., and more preferably from about 300xc2x0 C. to 400xc2x0 C.
In general, the poly(hydroxyamide-amic acid) precursors of this invention are soluble in common organic solvents, whereas the derivative poly(imide-benzoxazole) are not. Thus, in preparing PIBO articles, a precursor solution containing the poly(hydroxyamide-amic acid) precursor is formed into the desired shape of the finished product such as a film, a fiber or other article using conventional techniques before substantial conversion of the precursor to PIBO.
The PIBO copolymers of the invention may be cast into films useful as printed circuit backings, insulating dielectric layers and other applications where tough, flexible, high temperature stable films having good dielectric properties have been used in the past. In particular, the PIBO copolymers are useful as dielectric layers in semiconductor devices because they possess hydroxyl groups which can create better adhesion to semiconductor substrates such as silicon wafers.
The PIBO copolymers of this invention may be used as engineering plastics, molded using standard techniques such as compression molding or injection molding to produce melt-fabricated articles such as safety masks, windshields, electronic circuit substrates, or the like. They may also be compounded with fibers such as glass, graphite or boron fibers to produce molding compounds for high strength structural components such as jet engine components.
The PIBO polymers may be used as coatings on substrates such as aluminum or silicone dioxide. They are also useful to produce high temperature coatings for magnetic wire, dip coatings for various electronic components, protective coatings over glass, metal and plastic substrates, and photoresist coatings useful in microelectronic processing.
The PIBO polymers may also be used to produce high temperature adhesives for bonding electrical circuitry, conductive adhesives when mixed with conductive fillers such as silver or gold for microelectronic applications, or adhesives for glass, metal or plastic substrates. The PIBO copolymers of this invention may be melt spun to form fibers and filaments.
The polymers may also be used as varnish compositions or matrix resins to produce composites and laminates. The varnish compositions and matrix resins may be used to impregnate glass cloth, or graphite or boron fibers, for the production of printed circuit boards or other structural components requiring high temperature performance, non-flammability and excellent electrical properties.
Without intending to limit it in any manner, the present invention will be further illustrated by the following examples.
In these examples, 2,2-bis(3-amino-4-hydroxyphenol) hexafluoropropane (BisAPAF) was purchased from Chriskev Company. Trimellitic anhydride chloride was obtained from Aldrich Chemical Company and dried under vacuum at 60xc2x0 C. for 24 hours before use. Anhydrous N-methylpyrrolidinone (NMP) and pyridine were obtained from Aldrich Chemical Company and used without further purification.